1. Field of the Invention
This invention relates to an engine vibration sensor, more particularly to an engine vibration sensor which comprises a vibrator set to resonate at a predetermined, specific frequency of vibration of the engine to which it is attached, the vibration of the vibrator being converted into an electric signal representing the engine vibration.
2. Description of the Prior Art
Engines play an important role in ships, automobiles and many other devices and machines that contribute greatly to the quality of modern life. However, unless an engine is used under optimum operating conditions it is apt to suffer a decline in power output, abnormal vibration, abnormal wear and/or a decrease in fuel efficiency.
In order to assure that an engine is operating under optimum conditions, it is necessary to accurately monitor its actual operating state. One device known to be highly useful for this purpose is the engine vibration sensor. The usefulness of this device derives from the fact that the vibration of an engine at specific frequencies accurately reflects the operating state of the engine so that by measuring the magnitude and characteristics of the vibration at specific frequencies there can be obtained a considerable amount of data for use in optimizing the engine operating conditions. The specific frequency or frequencies selected for monitoring depend on the actual operating state of the engine. In the following, a specific example will be described in connection with the knocking vibration frequency of an engine.
Ordinarily, when ignition occurs too early in an engine, knocking and decreased power result. A decrease in power also occurs when the ignition is too late. It is therefore necessary to optimize the ignition advance so as to obtain maximum power and operating efficiency without causing the engine to knock. It is, however, no easy matter to determine the optimum value beforehand since it is dependent on the type of engine, the specific character of the engine, the number of revolutions and the intake pressure. Conventionally, the ignition advance has been set mechanically or electrically on the basis of the engine speed and the intake pressure. This method does not, however, always result in the optimum ignition advance. What has been done in actual practice, therefore, was to set the advance angle slightly smaller than the optimum value in order to prevent knocking, even though this meant that it was impossible to realize maximum power.
This expedient is no longer satisfactory since it runs counter to current demands for better engine fuel economy and fuel efficiency. The need for optimizing ignition advance is particularly strong in the case of the turbocharged engines that are being developed specifically for the purpose of reducing fuel consumption and boosting power. To meet the requirements of these engines there has been developed a knock control system for obtaining maximum efficiency wherein the ignition advance is automatically controlled using trace knocking as an index. For this system to operate effectively it is necessary to be able to measure the trace knocking of the engine quickly and accurately. Several types of engine vibration sensors have been developed and used for this purpose.
These known sensors include magnetostrictive, piezoelectric disk, piezoelectric cantilever and various other types, but none has been able to provide the required performance. For example, some are capable of precise measurement only within a limited temperature range while others can provide reliable results only at specific engine rotational speeds or under other specific measurement conditions. As a result, it has been difficult to carry out reliable engine knock control. Because of this, there has been desired a vibration sensor capable of reliably distinguishing between vibration peculiar to knocking and other miscellaneous types of vibration regardless of changes in the engine speed, temperature and other measurement conditions. Also, as most engines are of the multi-cylinder type, there has been desired a sensor capable of measuring the knocking vibration at a number of cylinders so as to make it possible to use a single sensor for optimizing the ignition advance for all cylinders of a multi-cylinder engine.
FIG. 1 shows engine vibration waveforms obtained by attaching a non-resonating vibrator having flat frequency characteristics to an engine and converting the engine vibration obtained through this vibrator to an electrical signal. The waveform shown in FIG. 1A is that obtained for an engine operating under a normal state of combustion without knocking while that shown in FIG. 1B is that obtained for an engine operating under an abnormal state of combustion with knocking. It will be noted from these graphs that in both cases the vibration wave periodically grows large in amplitude in synchronization with the combustion timing. In the case of FIG. 1B showing the waveform for an abnormal state of combustion, however, in addition to the periodic large amplitude waves there can be seen large amplitude vibration waves at positions somewhat shifted from the timing of the vibration peaks.
FIG. 2 shows the frequency spectra for the waveforms shown in FIGS. 1A and 1B. FIG. 2A shows the frequency spectrum for an engine operating under a normal state of combustion without knocking and FIG. 2B shows the frequency spectrum for an engine operating under an abnormal state of combustion with knocking. As is clear from these two graphs, the frequency spectrum in the case of normal combustion with no knocking is flat while that in the case of abnormal combustion with knocking is characterized by the occurrence of peaks at a specific frequency region.
Thus the vibration generated by knocking occurs within a frequency range extending from about 6 to 8 KHz while general vibration not related to knocking is spread over a wide range of frequencies. Because of this, by using a vibration sensor provided with a vibrator having resonant frequency characteristics which, as shown in FIG. 3, are coincident with the knocking vibration frequencies, it is possible to measure the engine's knocking vibration independently of its other general vibration.
In the case of the conventional vibrator sensors used heretofore, however, it is often difficult to make the resonant frequency of the vibrator coincide with the knocking vibration frequency and under some measurement conditions the resonant frequency of the vibration sensor will be different from the knocking vibration frequency. When the two frequencies fail to coincide, the sensor becomes incapable of distinguishing between the general vibration arising from various parts of the engine and the vibration peculiar to knocking, meaning that the measurement characteristics of the sensor will be degraded.
The causes behind this degradation in measurement characteristics can be divided into those arising from the vibration sensor and those arising from the engine.
The first cause is that the effective length of the vibrator element changes with the vibration. FIG. 4A is an illustration showing a conventional cantilevered vibration sensor, and FIG. 4B shows an illustration of a vibrator. In FIG. 4A, the root 12a of the vibrator 12 is clamped to the base 10 by a clamp 14 which is firmly fixed to the base 10 by screws 16. The retaining force by the clamp 14 forms such press mark 18 on the surface of the root 12a of the vibrator 12 as shown in black in FIG. 4B, and the press mark 18 is formed considerably back inside from the edge of the clamp 14 as shown in two dot chain line.
The effective length of the vibrator element 12b of the vibrator 12 is determined at the vibrator element side edge 18a of the press mark 18, and the resonant frequency of the vibrator element 12b changes inversely proportional to the square of the effective length of the vibrator element 12b. Accordingly, in order to fix the resonant frequency of the vibrator element 12b the vibrator element side edge 18a of the press mark 18 must be set definite and the effective length of the vibrator element 12b must be fixed. The vibrator element side edge 18a of the press mark 18, however, changes with the retaining force of the clamp 14, temperature and characteristic deterioration of the vibrator itself, the causes of which vary the resonant frequency characteristics of the vibrator element 12b. In addition to the above, the vibrator element side edge 18a of the press mark 18 changes with poor quality of the vibration sensor parts made in the manufacturing process, and the poor quality becomes the cause which varies the resonant frequency characteristics of the vibrator element 12b.
The second cause that can be mentioned in conjunction with the vibration sencer is the concentration of the retaining force of the clamp 14. As evident from FIG. 4B, the press mark 18 appears vaguely in the center portion and intensively on the peripheral portions of the root 12a of the vibrator 12. It is understood that the retaining force of the clamp 14 is concentrated on the peripheral sides of the root 12a of the vibrator 12. Since a piezoelectric element is attached on the surface of the vibrator element 12b for the vibrator sensor in most cases and the piezoelectric element is generally of fragile nature, there arise such drawbacks that the concentration of the retaining force causes cracks to change the resonant frequency characteristics of the vibrator element 12b.